Why the Engine Failure Could be Good News for SpaceX

SpaceX's Dragon capsule is on its way to the ISS, but not before losing one of the nine Merlin engines during the first stage of the Falcon 9 rocket. PM Contributor Rand Simberg explains why the engine failure can actually be seen as a positive.

SpaceX launched its first operational mission with the Falcon 9 last night, a cargo delivery to the International Space Station. While the ISS delivery seems to be going forward on schedule, the launch was not a complete success. There was an engine failure on the first stage during ascent, resulting in a slightly longer trajectory in terms of time, which may also have resulted in a slight disparity between the mission's planned orbital plane and its actual one. However, this isn't expected to affect the ISS mission significantly.

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While it was originally reported that the engine blew up, it now appears that it remained mostly intact. The company says that the visuals from the long-range camera show not an explosion, but rather an implosion. The engine reportedly started to lose pressure and fell out of specifications, which prompted the flight software to automatically shut it down.

When this happened, the pressure of the gases exiting the rocket nozzle went to zero. And it occurred at maximum dynamic pressure: the time when the rocket is going fast enough to cause a great deal of drag, but hasn't reached a sufficiently high altitude for the thinner air to reduce that drag. As a result, the pressure on the outside of the fairing outside the engine vastly exceeded the internal pressure, so the fairing collapsed inward, breaking off parts of it and perhaps parts of the nozzle itself. The power head of the engine itself (the part that would have the potential to "explode") is believed to have remained intact, because it continued to provide telemetry to the ground.

From an engineering perspective, all of this is good news for two reasons.

First, prior to this flight, the idea that the Falcon 9 could still get to orbit even with an engine out was just a marketing claim. Proving it would have required a demonstration in a test flight. Now that claim has been demonstrated and validated in an operational flight, if accidentally. While SpaceX's competitors and opponents will point to the engine loss as a reason for concern, in reality it should increase confidence in the company's product. Every rocket provider has problems (United Launch Alliance's Delta IV, one of Falcon's competitors, had a second-stage engine issue itself just last week), but in this case the design was sufficiently robust to overcome them exactly as the designers intended.

Second, if the engine really had exploded, this would have potential safety implications for a crewed version of SpaceX's Dragon capsule. Consider that the upper stage of the Falcon 9 uses the same Merlin 1C engine as the first stage, except with a few changes such as a larger nozzle for vacuum operations and passive radiative cooling rather than the "regenerative" cooling in the first-stage engines (it pumps fuel through channels in the nozzles to carry away the heat). The biggest difference, though, is that there is only one Merlin engine for the upper stage. So if it fails, the mission fails.

There have now been four flights of the Falcon 9, with ten engines each (nine for the first stage, one for the second). Counting the one engine failure from last night's launch, that means that the engine has a demonstrated operational reliability of 39 out of 40, or 97.5%. That means that there's a 2.5% chance that the engine would fail in an upper stage (where it has no backup), and would imply that this is an upper boundary on the reliability of the rocket itself (because other things could go wrong). This is in the typical ballpark of mission reliability for expendable launch vehicles for the past half century.

We've seen that SpaceX can endure a failure of one of the nine engines on the lower stage. But what would a failure of that solitary engine in the upper stage mean? It depends on the nature of it. But this question has a huge importance for SpaceX's goal of eventually launching humans into space.

If the engine shut down catastrophically, as was first feared about last night's incident, this would certainly result in a loss of mission and payload—unless the payload was a Dragon, as it was in this case. If it were a Dragon, and it had an abort system, then whether or not it would survive would depend on whether the shrapnel shield (if there is one) on the upper stage would prevent a rapid disassembly (likely explosion) of the stage itself, and how much warning the Dragon had before the event, to get away from it. How this scenario plays out would obviously be crucial to understanding the reliability and safety of the system for Commercial Crew, a topic very important to both NASA and SpaceX right now, with their recent contract award for that program.

If the engine shut down more gently, as it now appears happened last night, then a standard payload would be lost, but a Dragon would likely be recoverable with its internal payload intact (though not external ones, such as the OrbComm satellite that was a secondary mission last night.)

This is all to say: The fact that last night's failure was relatively benign should reassure not only pressurized payload customers, but also potential future passengers of the vehicle. But the analysis is almost moot at this point, because after the next planned flight, Falcon 9s will be using the upgraded version of the engine, the Merlin 1D, giving it a clean slate for operational reliability data. It is quite possible that whatever SpaceX cites as the cause of the engine failure on last night's flight to be will be solved simply by the upgrade, so it's hard to know what the reliability of the vehicle will be going forward for either crew or cargo. What we will know, though, is that SpaceX's redundant-engine design approach, while possibly degrading schedule reliability, has been validated by launch reliability.

Unfortunately, the bad news for the company from a customer-relations standpoint is that while its current ISS mission won't be affected significantly, the aforementioned secondary OrbComm mission seems to have failed, at least so far. The Falcon was supposed to deliver the satellite to orbit, and it did. But it delivered the satellite to the wrong orbit due to an inability to relight the second stage after separating from the Dragon—itself a result of ISS rules imposed by the mission's imperfect orbit insertion. Whether this is a temporary or permanent setback for OrbComm is unknown, but the company is still hopeful, and it will no doubt be part of the story in days to come. At the very least, it has said it plans to fly on Falcon 9 again.